This invention relates generally to the field of hydroelectric turbines that produce electricity by harnessing the flow of water, and more particularly relates to such devices wherein the tidal flow of water causes rotation of a large impellor-type rotor having an annular outer rim disposed within a large annular housing.
While most turbines are constructed to have a central rotating shaft onto which the blades or runners are mounted, it is also known to produce open-centered turbines, also known as rim-mounted turbines. Turbines having open-centered rotors, where the blades are mounted between inner and outer annular rings or rims and where the energy is transferred through the outer rim to an annular housing that retains the rotor, can be particularly successful in low head conditions, i.e., in slower currents.
Examples of open center, rim-mounted turbines can be seen in U.S. Pat. No. 5,592,816 issued Jan. 14, 1997, and reissued as RE38,336 on Dec. 2, 2003, U.S. Pat. No. 6,648,589 issued Nov. 18, 2003, U.S. Pat. No. 6,729,840 issued May 4, 2004, and U.S. Patent Appl. Publication US2005/0031442 published Feb. 10, 2005 (Ser. No. 10/633,865). Examples of hydroelectric turbines used in low head (tidal flow) conditions can be seen in U.S. Pat. No. 4,421,990 to Heuss et al., U.S. Pat. Nos. 6,168,373 and 6,406,251 to Vauthier, UK Patent Appl. No. GB 2,408,294 to Susman et al., and WIPO International Publication WO 03/025385 to Davis et al.
Tidal powered turbines are seen as environmentally safe replacements for electrical power plants that utilize fossil fuels or atomic energy. In harnessing water to produce electricity on a large scale capable of powering industrial complexes, towns, cities, etc., it is necessary to provide large numbers of turbines, and it is necessary that the turbines be as large as practical in order to maximize the amount of electricity produced by each turbine. The rotor blades of these turbines are multiple meters in length, with some experimental designs having blades exceeding 50 meters in length.
As the length of the rotor blades is increased, structural and manufacturing challenges are presented that are not encountered in smaller turbines or generators. For shaft-mounted turbines, it is difficult to provide long blades that are both strong and light. In one solution, the blades of the shaft-mounted turbine are provided with an outer annular rim, which is contained within an annular housing, thereby providing support to the blades through the shaft and the rim. Alternatively, rim-mounted turbines with no central shaft provide a solution to this problem by providing annular support to the inner and outer ends of the blade, with the outer support rim being retained within a housing having an annular slot or channel. In a typical means for generation of electrical power, a large number of magnets are spaced along the annular support rim and a large number of coils are spaced along the receiving channel in the stator housing. The magnetic field established by the rotor field system passes across the gap that separates the rotor and the stator. Rotation of the rotor causes the magnetic flux linkage with the coils to change, inducing an electro-magnetic force in the coils.
In order to reduce the start up torque of such open centre turbines, and often due to manufacturing tolerances, such turbines are normally produced with the annular slot or channel in the stator being greater in diameter than the rotor. This results in the rotor effectively having a floating axis of rotation, meaning that the rotor is not fixed concentrically within the stator, and can undergo a degree of movement, float, and/or eccentric rotation within the stator. Furthermore, the axial thrust bearings have a clearance allowing the rotor to move axially and for its plane of rotation to depart from being parallel to the plane of the stator so that the rotor motion may contain precession elements and other complex patterns. This eccentric rotation can however lead to unbalance in the voltage generated in the coils, with the coils on the stator that are in closer proximity to the magnets on the rotor, due to the eccentricity of the rotor, generating a disproportionate amount of EMF.
When a number of such coils are connected together in parallel, their differing induced EMFs result in the coils carrying different current and the coils do not share the electrical load equally. A small displacement of the rotor from the concentric position can lead to a disproportionately large inequality in the distribution of load. This can leave the coils susceptible to overheating and to reduced efficiency of power conversion.
It is therefore an object of the present invention to overcome the above mentioned problem without recourse to the difficult and expensive solution of using high-tolerance, closely-fitting bearings to support the rotor in a concentric co-planar position within the stator.